1. Field of the Invention
This invention relates to drive mechanisms for door hardware, such as drive mechanisms for retracting the pushbar of an exit device or remotely locking a door lock. More specifically, the present invention relates to drive mechanisms that include a sensor for detecting motion of the door hardware component being driven.
2. Description of Related Art
Door hardware, such as exit devices, mortise locks and bored locks, typically include one or more elements that move between two positions, such as a retracted position and an extended position. For example, a pushbar exit device includes a pushbar that moves inward to retract a latchbolt from a strike in a doorframe and outward to extend the latchbolt. Lock mechanisms include a handle, a latchbolt and other locking elements that may be driven between two alternative positions. The moving lock component may be a locking element that locks and unlocks the door or it may be a latchbolt that latches and unlatches the door, etc.
Where it is desired to operate the door hardware remotely, the drive mechanism typically includes a driver that is electrically powered. The driver may be a conventional DC or AC motor, a linear actuator, a stepping motor or any other known device for providing mechanical motion from electrical power. In a typical design, the door hardware component is spring biased towards a first default position and the driver acts against the spring force to move the driven component towards the second position. When the driver is turned off, the spring returns the moving component to the default first position.
For convenience, the present invention will be described in the context of an exit device where the moving door hardware component is a pushbar mounted on a pair of rocker arms in a conventional parallelogram linkage mount. The pushbar is spring-biased towards an outwardly extended position and can be driven or manually pressed to the inward position to open the door. The driver is a linear actuator that includes a stepping motor and a threaded output shaft. When the driver is operated, it pulls on one of the rocker arms and moves the pushbar in towards the door against the spring biasing force. The pushbar, in turn, retracts a latchbolt from a strike in the door and unlatches the door.
It should be understood, however, that the present invention may be used in other types of door hardware, including mortise locks and cylindrical or bored locks and may be used wherever a door hardware component is driven between two alternative positions.
Electrically operated exit devices of the type described herein are often used in schools or public buildings where they are opened and closed on a fixed time schedule at the start and end of the day. Remote unlocking and opening of exit devices may also be desired for keyboard access to improve wheelchair access or for control by a remotely located security guard.
Conventional electrically operated door hardware has typically mechanically connected the driver directly to the moving component. When the driver is commanded to move, the mechanical output of the driver directly moves the door hardware component to the desired position. A difficulty with this design arises when the door hardware component being driven is blocked and prevented from moving.
For example, where the driver includes a stepping motor and the pushbar is temporarily blocked, the stepping motor may slip and fail to move when commanded by the controller. The controller, however, may believe that the door component has been moved. As a result, the driver fails to move the component to the correct final position, which may leave the door locked when it should be unlocked.
To resolve this temporary blockage condition, it may be necessary to completely reset the entire door locking system. Resetting all door hardware in a large system, such as in a school where multiple doors are under common control, is undesirable as it disrupts access to the entire building. On the other hand, individually resetting a single door each time this occurs is time consuming and expensive. Someone must be sent to reset the individual door each time this temporary condition occurs. A system that detects temporary blockages and automatically resets would provide improved performance.
Direct drive designs of the type described above typically drive from a known starting position (set by the default, released, spring-biased, outward position of the driven component) to a final position a predetermined driven distance away from the starting point. Attempting to reach a final position by driving a known distance from a starting position can be problematical. In some cases the desired final position is not known until the product is installed. In other cases, wear may change the desired final position. Alternatively, temporary blockages, motor slippage and the like may prevent the component from reaching the desired final position, even though the controller believes the final position has been reached.
Another approach is to place a single sensor at the final position to detect arrival of the component at the final position. This can also be problematical, as the desired final position may change for the reasons described above. A design that automatically detects that it has arrived at a desired final position would be desirable, even where the final position changes over time or in different installations.
A related problem in conventional designs is mechanical shock sensitivity. If door hardware is subjected to a mechanical shock, as occurs when an open door slams closed in a windstorm, some drivers, such as those that include a stepping motor, may completely release. This release is caused when the mechanical load imposed by the shock exceeds the holding force supplied by the stepping motor. When this happens, the controller loses track of the location of the moving door hardware component, causing incorrect operation. A system that reduces mechanical shock to reduce errors of this type would also provide improved performance.
Another desirable feature would be a system that automatically calibrates itself so that the system automatically adapts to different installations, automatically adjusts for wear, compensates for some errors in manufacturing and/or can be used in different designs of door hardware without modification.